City officials fired a Gladstone police sergeant implicated in a murder-for-hire plot because of potentially criminal misconduct unrelated to the 2011 death of his estranged wife, according to a document obtained Wednesday by The Oregonian.

In a Dec. 12 memorandum, Police Chief Jim Pryde says Sgt. Lynn Benton engaged in a "fraudulent marriage" to a Brazilian man in 1993, when Benton was legally female, solely so the man could qualify for U.S. citizenship, potentially a violation of federal law.

The memo also says Benton possessed and viewed pornography on his police-issued laptop computer, a violation of city policy.

The misconduct issues were investigated by Portland and Milwaukie police at Pryde's request. Both allegations were "sustained," Pryde's memo says.

"As a result of these findings, bearing in mind the gravity of each individually and collectively, it is no longer tenable for Lynn Benton to serves as a police officer with the city of Gladstone," Pryde wrote. "Therefore, I am recommending immediate termination."

City Administrator Pete Boyce fired Benton Dec. 21 after a hearing but declined to release the results of the internal investigation. The Oregonian appealed the city's decision to withhold the document to the Clackamas County District Attorney's Office, which ordered the release.

"Given Benton's status as a high-level officer and the basis for discipline, the public interest compels disclosure," Deputy District Attorney Sarah Dumont wrote in a Jan. 25 order to release the memo.

Boyce declined Wednesday to comment, saying additional discussion could violate the state's public records law and expose the city to liability.

Benton, then named Lynne Irene Benton, married Jose M. Zonis, a Brazilian immigrant, in 1993. They divorced in 1996, according to Clackamas County court records. Zonis, contacted in Minneapolis for a previous article, declined to answer any questions about their relationship.

Marrying solely to qualify for citizenship violates federal law. However, no charges have been filed against Benton or Zonis.

Benton was placed on paid administrative leave the day after his wife, Deborah Higbee Benton, 54, was found slain in May at her Gladstone beauty salon. Benton was paid $6,357 a month during the four-month investigation that resulted in his dismissal.

Benton, 49, was implicated as a co-conspirator in his wife's death during a December bail hearing for murder defendant Susan Ellen Campbell. Clackamas County prosecutors said Benton, a 24-year police veteran, offered Campbell $2,000 to kill Higbee Benton.

However, Benton has not been charged. After the hearing, Gregory D. Horner, Clackamas County chief deputy district attorney, said, "the investigation is continuing. We're not going to comment any further."

Meanwhile, Campbell, 54, is being held without bail in the Clackamas County Jail. Her trial is set for March.

Prosecutors said in the December bail hearing that Campbell went to Higbee Benton's salon on May 28 and shot her in the back with a .25-caliber semiautomatic handgun. But the shot wasn't fatal. Campbell immediately called Benton, who was working an overtime shift at police headquarters, prosecutors said. Investigators later found .25-caliber ammunition in Benton's police locker.

An autopsy indicated Higbee Benton was severely beaten, suffering a dozen broken ribs, a lacerated liver and evidence of strangulation, including a fractured thorax. The medical examiner characterized it as an act of "kill, kill and overkill."

Benton and Higbee Benton married in 2010 in what became a tumultuous relationship, prosecutors said. Before the marriage Benton underwent a female-to-male sex-change operation and had legally become Lynn Edward Benton.

Prosecutors said Higbee Benton initially supported the gender change, then opposed it, leading to Benton moving out of their home in April.